Filters, Grids, and Collimators PDF

Filters, Grids, Collimators Filters How is filtration measured to measure filtration, we use the half-value layer - which measures amount of material required to attenuate the XR to 1/2 the original output....

Filters, Grids, Collimators Filters How is filtration measured to measure filtration, we use the half-value layer - which measures amount of material required to attenuate the XR to 1/2 the original output. This is measured in mm Aluminium What are filters and their purpose? Filters are metal sheets placed in XR beam between the window and patient that are used to attenuate the low-energy XR photons from the spectrum. These XR not only do not contribute to the quality of image but also increase entrance skin dose (ESD, patient dose). What are the types of filtration inherent and added What is inherent filtration and what is it made up of? How does inherent filtration differs for imaging modalities that require low-dose XR features of XR tube and housing design which filtrates XR beam before the beam exits the XR machine traditionally, made up of glass envelope, insulating oil, housing window always present (because its part of the machine) reduce entrance ESD to patient low energy tube: inherent filtration is minimised. Made from low Z material such as Be For specialised low energy application (e.g., mammography) - k-edge filters such as Mo or Rh are used What are used as added filtration? What are their effects? Filters, Grids, Collimators 1  Added filtration are usually in the form of interchangeable metal sheets (Al, Cu, etc.) Usually 2-3mm of Aluminium Function remove unwanted low energy XR to reduce patient dose remove unwanted high energy XR to maximise image quality (reduce scatter) modify the spectrum of XR beam to optimise paediatric or mammography imaging produce uniform density image in cases where there is a large range of tissue thickness across the patient using wedge filters and compensating filters What is total tube filtration? What is the minimum total filtration (mm Al) for a 80kVp generator? Tube filtration = inherent + added filtration Total filtration must be 3 mm Al for at 80 kVp generator What are the minimum tube filtration requirements for an XR machine (in HLV)? Minimum tube filtration requirements (using HVL): for a beam < 50kVp, total filtration (HVL) must be 0.5 mm Al total filtration must be > 2.5mm Al for >110 kVp generator Filters, Grids, Collimators 2  What is the minimum inherent filtration of an XR tube in HVL? >0.5mm Al or 0.03mm Mo How does filtration affect the XR spectrum graph X-Ray Production What is the effect of filter on skin dose less exposure to skin Filters, Grids, Collimators 3  Explain what is K-edge filtering and application? Uses filtration materials that a selective k-edge (e.g., Mo @ 20 keV) This removes both (1) lower enegy Bremstrahhlung protons that do not contribute to image -> increase beam quality, reduces patient dose and (2) high energy photons just above the k-edge => increased chance of P.E interaction Used in mammogram e.g., Mo @ 20 keV => signification proportion of Mo/Mo spectrum is 17.4 - 19.6 keV What are compensating filters, name some that are commonly used and when they are used? When an anatomical part varies substantially in tissue density from one area to another (e.g., chest), it is possible to add filters to enhance visibility of entire structure without having to make additional exposures to thicker regions compensating filters function to reduce radiation reaching the thinner, less dense areas ⇒ produce uniform density images wedge vs trough filter (trough for chest) Filters, Grids, Collimators 4  Explain how contrast agent works Filters, Grids, Collimators 5  Collimation What are collimators? Collimators are XR beam restriction device attached to the housing of XR tube it is a metallic barrier (usually lead) which reduces size of XR beam What are the effects of collimators on image quality? more collimation = decreased field of view (FOV) by narrowing beam width increasing collimation → narrowing beam width → decreased FOV → irradiate smaller volume of tissue → less scatter interactions → better contrast Grid (Anti-scatter Grids) What are grids? device that contains multiple thin metallic (usually lead) strips with interspersed radiolucent spaces (holes) → forming a grid that reduce the amount of scattered radiation that reaches XR detector Filters, Grids, Collimators 6  they are placed behind the patient, in front of the image receptor scatter = does not originate in a straight line from focal spot → grids remove beams that travel in an angle (presumed scaterred XRs) preferentially remove scattered radiation What is the grid ratio and what is the grid frequency? grid ratio = ratio of height of lead strips to distance between each strip (= h/D) h = height, D = distance between each strip grid frequency = 1/(t+D) - number of grid lines per unit distance (either inches or centimeters) t = thickness of lead strip D = distance between each strip 1 = 1cm. However lead width are usually in micrometers ⇒ 1cm = 10,000 micrometers ⇒ formula = 10000/(t (in micrometer) + D) How do grids contribute to image quality and dose? improves contrast by reducing scattered radiation (mainly contributed by Compton interaction) dose is increased (higher grid ratios require more technique to penetrate (Bucky factor), which increases the overall patient dose) What are the types of grids Filters, Grids, Collimators 7  Based on orientation (4): linear parallel linear focussed: strips inclined towards midline linear crossed: : 2 linear grids perpendicular to each other focussed crossed: strips are inclined towards a focal point Based on movement stationary moving: moving the grid helps make the grid lines invisible on the film How is grid performance calculated (or what is the parameter of grid performance). Grid ratio: what is it? what happens with increasing grid ratio? height of grid/width of interspace material Higher grid ratio removes more scatter BUT increase pt. load (as more mAs is required to penetrate) increase tube load (as more beams are attenuated) need to be positioned carefully due to risk of grid cut off (undesirable absorption of primary radiation by grid) Contrast improvement factor - what does it measure, what is formula, what does it depend on? Filters, Grids, Collimators 8  measurement of the ability of grid to improve contrast reflects increased image quality obtained from grid use contrast improvement factor (K) = radiographic contrast with grid/radiographic contrast without grid most grids have value between 1.5 - 2.5 Depend on: Scatter Patient thickness → thicker patient, higher CIF Area irradiated → larger area, higher CIF basically increase in all these factors increases compton scatter → more scatter to remove → higher CIF XR spectrum → higher energy, higher CIF Grid characteristics Bucky factor - explain what is the Bucky factor? Ratio of incident XR / transmitted XR through a grid It reflects the radiation dose penalty due to the use of a grid Bucky factor of 2 → patient will receive double the dose What is the typical grid ratio vs mammography grid ratio typical: 10:1; mammography: 5:1 (height : interspace) What is the grid cutoff? when so many photons are blocked that you cause a quantum mottle → noisy image happens when grid is placed incorrectly What is the air gap technique? what is the purpose? method similar to grid - reduce scatter separating the patient from film/receptor Filters, Grids, Collimators 9  in this technique, scatter is not reduced via filtration but by radiation missing the target (due to presence of space between patient and image receptor) Grid selection high kVp techniques should use grid with high grid ratio as high kVp → Compton effect dominates → more scatter thicker body sections require grids with high grid ratio as thick → more Compton → more scatter how does grid ratio affect CIF increase grid ratio → increase CIF explain pros and cons of heavy grids pro: high CIF, high selectivity → more efficient at removing scatter con: increase patient dose Lecture Note Lecture Notes - Topic 5 (Filters, Collimators and Grids) Role of filters, grids and screens in imaging: creating optimum image with necessary clinical information with lowest radiation dose to pt. Filters to measure filtration, we use the half-value layer - which measures amount of material required to attenuate the XR to 1/2 the original output. This is measured in mm Aluminium There are 2 types of filtration inherent filtration - Filters, Grids, Collimators 10  features of XR tube and housing design which filtrates XR beam before the beam exits the XR machine traditionally, made up of glass envelope, insulating oil, housing window always present reduce entrance ESD to patient low energy tube: inherent filtration is minimised. Made from low Z material such as Be For specialised low energy application (e.g., mammography) - k- edge filters such as Mo or Rh are used added filtration from interchangeable metal sheets (Al, Cu, etc.) remove unwanted low energy XR to reduce patient dose remove unwanted hihg energy XR to maximise image quality (reduce scatter) modify the spectrum fo XR beam to optimise paediatric or mammography imaging produce uniform density image in cases where there is a large range of tissue thickness across the patient using wedge filters and compensating filters Tube filtration = added + inherent filtration Minimum tube filtration requirements (using HVL): for a beam < 50kVp, total filtration (HVL) must be 0.5 mm Al total filtration must be > 2.5mm Al for >110 kVp generator Filters, Grids, Collimators 11  XR Tube filtration total permanent filtration (inherent): >0.5mm Al or 0.03mm of Mo beam quality is defined by HVL HVL for diagnostic XR unit: >2.1mm Al at 70 kVp How filter affects XR spectrum energy curve: Filters, Grids, Collimators 12  effect of filter on skin dose higher filtration → less exposure dose to skin limit on filter thickness above a certain thickness there will be: little gain in dose reduction increased tube loading longer exposure times increased risk of motion artifacts target/filter combination (mammography) used in mammography refers to combiation of target and filter materials window of mammography x ray tube should be beryllium (not glass) with maximum thickness of 1mm - minimise filtration typical target/filter combinations nowadays are: Filters, Grids, Collimators 13  Mo + 30um Mo, Mo + 25um Mo W + 60um Mo W + 50um Rh W + 40um Pd Rh + 25um Rh compensating filters When an anatomical part varies substantially in tissue density from one area to another (e.g., chest), it is possible to add filters to enhance visibility of entire structure without having to make additional exposures to thicker regions compensating filters function to reduce radiation reaching the thinner, less dense areas wedge vs trough filter (trough for chest) Filters, Grids, Collimators 14 Filters, Grids, Collimators 15